This invention relates to programmable logic devices, and more particularly to programmable logic array integrated circuits with spare circuits for replacement of other circuits which may be defective.
So et al. U.S. Pat. No. 4,899,067 shows programmable logic devices having sum-of-products logic which is subdivided into relatively small units called macrocells. In order to salvage parts which may have been defectively manufactured, the So et al. patent shows including extra macrocells (or extra word lines or bit lines which form portions of the macrocells or the structures used for interconnecting the macrocells). When a macrocell (or a word line or bit line) is found to be defective, the defective circuitry is switched out of service and the spare circuitry is switched into service to make up for the capacity that would otherwise be lost. In this way parts which would otherwise have to be scrapped can be salvaged and used exactly as though they were defect-free.
The So et al. invention works extremely well. However, because the replacement of defective parts takes place on the relatively small-scale level of individual macrocells (or individual word lines or bit lines), a relatively large amount of additional control circuitry is required. For example, in products of the size described in the So et al. patent (e.g., the so-called Max 5000 EPLD available from Altera Corporation of San Jose, California) over 2000 fuses are required to control the use of the spare circuitry. This is a substantial "overhead penalty" to allow defective parts to be salvaged and used. Also, while "repair" on a relatively small-scale basis (as in the So et al. patent) is good for "point" or relatively small defects, it would be desirable to have a strategy for repair on a larger scale, which strategy would be more effective for repairing larger or more severe defects.
In view of the foregoing, it is an object of this invention to improve and simplify the provision and use of spare circuits in programmable logic devices.
It is another object of this invention to provide programmable logic devices with spare circuits for replacement of defective circuits on a relatively large scale so that the overhead associated with controlling the use of the spare circuits can be substantially reduced and so that larger or more severe defects can be more effectively overcome.